How Ravenscroft discovered English Lead Crystal

Aditional copy uploaded 25.05.2010.

The Ravenscroft seal. Diam. c. 1 cm.What has become one of the great unsolved mysteries in glass history is how George Ravenscroft, an Englishman generally thought to have had no technical knowledge of glassmaking, came to invent a new form of crystal glass that surpassed in its qualities the known white glass of the time. The matter is all the more surprising when it is appreciated that there were in London, where the discovery was made, no less than six glasshouses making, or capable of making, white glass at that time. That at Greenwich was described by diarist, John Evelyn in 1673 – the year that Ravenscroft took out his patent - as blowing glass “of a finer metal than that of Murano in Venice.” Another, in Red Mead Lane near the Tower of London, belonged to the King’s brother, the Duke of York, later King James II. The competition was fierce for a relatively small upper-class market, for white glass was very expensive. And then there was the question of competition from Venetian imports by merchants such as John Greene. Starting up a glasshouse was an expensive business; it was not an area for research into which a merchant whose known experience related mainly to selling high class goods was likely to venture.

A further consideration involving royalty was that only a few years earlier King Charles II had saved Ravenscroft from disaster in Venice. The matter had involved a fight between George and his younger brother, Francis, with the greatly disliked British ambassador. The Venetian Senate treated what might seem to have been a minor skirmish with extreme severity and passed the sentence on the brothers of “banishment from Venice, and all Venetian territories and ships.”

Portrait of King Charles II wearing a lace ruff compared with a sample of point lace made in the Venetian island of Burano: Venetian point lace was one of the luxury items imported by George Ravenscroft to supply royalty and rich merchants. It should have effectively terminated Ravenscroft’s trade there, but through the intervention of a titled Lady, the Duchess of Modena, King Charles II of England came to the rescue and the penalty was overturned. A little later, when Ravenscroft was in trouble with the Venetian bank over it withholding his funds, the King again intervened by personally writing (in Latin to make sure he was understood) to the Doge. Such involvement by the King suggests a very personal relationship not normally open to a mere merchant.

The royal concern was probably that Ravenscroft imported very expensive point lace from Burano, another of the Venetian islands, that can be seen adorning the cuffs and collars in royal portraits. For Ravenscroft, such patronage was not to be treated lightly and a direct venture into research for improving crystal glass that might conflict with the King’s interests, such as that at Red Mead Lane in the London Docks area, if it was ever contemplated, might not have been received with approval.

Map of Venice showing the islands of Murano and Burano where the lace was made.Catholic historian, Rosemary Rendel, who provided much of this information, was of the view that the real concern of the Senate was the Ravenscroft family's growing involvement with glassmaking in Murano. There was some suggestion that George was involved in smuggling unpolished glass plates, used for carriage windows etc., out of Venice. The practice was illegal and would certainly have rung alarm bells with the Senate. What made matters worse was that one of the brother’s, probably Francis, is thought by Rendel (although I have not seen the evidence) to have gone from his initial training in Flanders to Florence and worked in a glasshouse in the Boboh Gardens there and had now tried to work in Murano itself. (Francis briefly took over, or proposed to take over, the Savoy glasshouse after George's discovery of English lead crystal.)

Overall, this more detailed evidence explains the Ravenscroft commercial interest in glassmaking but mitigates strongly against the idea generally prevailing among English historians from the 19th century to the present day that Ravenscroft deliberately undertook research that led to the discovery of English lead crystal (Table 1). In fact, there is absolutely no evidence to support such an approach.

Table 1. Some major proponents of the "Experimental" theory of how Ravenscroft “Invented” English lead crystal glass. Not one of them is supported by evidence.

"The total replacement of carbonate of lime by oxide of lead was not . . . as claimed by Mr. Hartshorne, the result of a sudden invention by Powlden or Tilston or Ravenscroft but of successive tentative experiments to make a more readily fusible glass." (Harry J. Powell, Glass Making in England, 1923, p.32.)

"Ravenscroft set up a glasshouse in the Savoy in July 1673, and seems to have spent at least a year in preliminary experimental work . . ." (W.A. Thorpe, A History of English and Irish Glass, 1924, reprinted 1968, p. 119.)

"It is thought that George Ravenscroft was, after a series of experiments, the inventor of the typical British flint glass." (Francis Buckley, Old English Glass, 1925, p.24)

"Ravenscroft's glass-of-lead was no sudden invention, but the culmination of a series of experiments..." (G. Bernard Hughes, English, Scottish and Irish Tableglass, 1956, p.43)

"Ravenscroft . . . at his experimental glasshouse in the Savoy, London, he set about trying to find a new sort of glass resembling rock crystal." (Ruth Hurst Vose, Glass, 1980, p.118.)

"George Ravenscroft when he started to experiment with the production of lead glass for vessels . . ."
(Susan Frank, Glass and Archaeology, 1982, p.83.)

This article is directed to questioning my earlier view that the timescale between setting up the furnace and applying for a patent was, on scientific grounds, too short to allow for an experimental approach and must therefore result from an accidental discovery. Her conclusion was that as a result of experiments "...in 1675-76 Ravenscroft chanced on lead oxide as an alternative to a large part of the salts." In other words an accident but no explanation of how. (Christine Macleod, Accident of Design? George Ravenscroft’s Patent and the invention of Lead-Crystal Glass, 1987, Society for the History of Technology, p.776.)

"Experiments brought to fruition by George Ravenscroft . ." (Hugh Tait, Five Thousand Years of Glass, 1991, p.182)

"At this point George Ravenscroft . . . decided to set up a glass furnace, with workers from Murano, at the Savoy in London." (Robin Hildyard, in Glass (ed. Reino Leifkes), 1997, p.88.)

"The presumption is that Da Costa, Reinier and Odacio experimented with lead glass in some way, but surviving documentation is unenlightening." (Peter Francis, The Development of Lead Glass: The European Connection, 2000, Apollo, p.47.)

How good was the Venetian Cristallo?

This is a question that advocates of the experimental hypothesis have not considered. The sixteenth century recipe books reveal that considerable trouble was taken by the Venetian glassmakers to obtain the purest possible ingredients for their best cristallo. First, they had a monopoly on the best ash obtainable, so called "aluma catina" from Syrian shore-growing plants. This was then purified by dissolving the finely powdered ash in hot water, filtering throught a double layer of fine cloth to remove any sediment and then evaporating the resultant lye and collecting with a perforated wooden spoon the crystals of sodium carbonate as they formed. A well-tinned boiler to prevent any possible contamination from iron is specified for this purpose. As I have found from personal experiments a good white crystalline solid is the result.

The other ingredient, powdered river pebbles, have the benefit of being very low in iron because the oxide adheres only to the surface of the pebble. Sand, by comparison, has a very much higher surface to volume ratio and is proportionally more contaminated by iron etc. That seen by Robert Hooke to have been used by da Costa is described as "calcined flints as white as flour." I have seen one recipe where even this procedure was improved upon by first boiling the flints in vinegar which would dissolve off a large part of any iron that was present.

These were the only two ingredients used to make best cristallo. They reflected well over a century of development and experiment by the Venetian craftsmen who were also well aware of the need for a clear and smokeless flame in the furnace. In use, a finely powdered and sieved mixture of the two ingredients was partly roasted to form manageable lumps called materia prima. This materia prima (called frit in England)could be stored and used as required in other recipes.
Part of the reason for taking this trouble was the need for only the best ingredients to make enamels that that would fuse to gold and silver or for making false jems. Neither were made by English glassmakers at that time. It is difficult to see how any improvement could be envisaged.
It should be mentioned that for common Venetian glass the ash had the rubbish picked out but apart from crushing and sieving was not further purified. However, crushed flints were still used to give a respectable although slightly grey glass.

English glassmakers, by comparison, laboured under the problem of smoke and soot from the coal-fired furnace. To counter this saltpetre was added to the batch, a practice dating back to Mansell in the first quarter of the century. From historian Robert Plot's remarks it seems that the use of crushed pebbles was unknown prior to da Costa. Verselini (1522-1606), must have been aware of the Venetian tricks of the trade but for colour his glass does not bear comparison. Only the Greenwich glasshouse is reported by John Evelyn in 1673 to have made glass of a quality comparable to that of the Italians.

We might think that Ravenscroft, rather than experimenting to make crystal, harboured an ambition to make false jems and bijouterie, an achievable possibility which would certainly fit in with his merchant trade. He might even have initially employed da Costa with this in mind. Could this explain why da Costa was making calcedonio?

So, if deliberate experiment is excluded what chance event can we identify with the discovery of English lead crystal?

Continental Lead Glass

Possibly the earliest documented use of lead glass is by Heraclius, in the 12th - 13th century, for the manufacture of coloured enamels. By the mid-16th century when Italy was just beginning to recover from the Black Death of 1339 that killed an estimate half of the population, we find a number of Venetian recipe books that include the manufacture of lead glass and its use for coloured glasses. The devastating experience of the Black Death had perhaps inspired artisans to record their secret processes for posterity against the possibility of another such catastrophe.

The lead glass described was not lead glass in the sense that we know it today but simply a fusion of lead oxide and ground flints with no added alkaline flux – essentially giving the chemical compound, lead silicate which has a natural yellow colour. Used on its own, or even with some added frit, this form of lead glass does not give a white (colourless) glass.

Moretti & Toninato in their Recipes of the Renaissance (2001) compare those of five authors between 1536 and 1644. All use this same mixture of just lead oxide and crushed flints although the ratio lead oxide:flints varies from 1:1 to 3:1 except with Neri who quotes 4:3 (which may explain why he states that this lower lead but higher melting glass can be used to make tableware.)
Colin Brain in Corning’s Glass of the Alchemists (2008) explains how it remained the glass additive used in Europe throughout the 17th century.

The recipes indicate that it was mostly produced on a small scale relating to its use for false jewels, bijouterie, enamels and teserae. A major problem with this glass is that due to the absence of an oxidising agent molten metallic lead is produced during founding. This not only destroys the pots but was very difficult and expensive to eliminate. “Tis a thing unpracticed in our furnaces because of the exceeding brittleness thereof.” wrote Christopher Merrett in 1662; nor was it normally practiced anywhere else except as part of other recipes for the specific manufacture mentioned above for which small pots were used. One such jewel or bijouterie maker was John Baptiste da Costa and it is his meeting up with Ravenscroft that led to the discovery of English lead crystal.

The Discovery of English Lead Crystal Glass

The solution to the discovery of English lead crystal emerges through what must have been a chance encounter between Ravenscroft and da Costa, a Jewish glassmaker from Piedmont in Northern Italy. How this happened we do not know but da Costa had rich relatives who were bankers in London (later thought erroneously to have founded the Bank of England). Ravenscroft might have used da Costa’s bank for his Venetian transactions. We first learn of John Baptiste da Costa at a glasshouse in Nijmegen, Holland, in 1665, along with two colleagues, de Reinier and Formica where Irish historian, Peter Francis assumes that they invented English lead crystal. But, in 1668, the City council replaced them by two competitors. De Rienier moved to Rotterdam where he was shortly joined by da Costa to make “Venetian crystal” and “fine and large wine and beer glasses.” There is no mention of a new form of lead glass!

Francis is misleading over the evidence concerning Formica. He is not included in lists of other glassmakers in Holland at the time and appears only to have been involved in a transaction concerning the Nijmegen glasshouse in 1665. We know only that he turns up later in Ireland. Contrary to what one might expect, in the intervening eight years until da Costa had met up with Ravenscroft there is no practical demonstration or reference to the exciting and profitable invention of a new lead crystal by any member of the trio.

In London, as Ravenscroft spoke both Italian and Latin, there was no language barrier when he met da Costa. It was perhaps their common interest in glassmaking that brought them together. Da Costa must have told him of his speciality in making bijouterie (small decorative items) and Ravenscroft must have recognised an opportunity for profitable sales alongside the outlets for his point lace that would not conflict with any royal interest. Rocque's 1746 map showing the location of the Savoy Palace fronting the River Thames between the cities of London and Westminster.This is speculation but is supported by evidence that we next learn that Ravenscroft (not da Costa) has built a small furnace with two chairs (unlike the full-size glassmaking furnace suggested by Francis) at the old Savoy palace where he has set da Costa to work. Building a furnace would not have been easy for a foreigner but Ravenscroft would have had little problem in getting royal approval, sourcing the site, materials and fuel and arranging a contract with the monopoly body of the Glass Sellers Company if necessary. For example, G. Bernard Hughes tells us that "He paid a Mr. Gray sevenpence a bushel on the spot (for fireclay) and was responsible for the cost of carriage by canal to Bewdley, by river to Bristol, and thence by sea to London."

It is pertinent to point out here that contrary to several accounts Ravenscroft did not employ any technicians or assistants from Venice for the Savoy glasshouse. Da Costa, the only employee, was not a Venetian.

It has been generally overlooked (or ignored) that da Costa, working in the Savoy glasshouse, was not at that time making English lead crystal invented in Nijmegen (or even ordinary glassware) as supposed by Francis, but the Venetian decorative speciality of calcedonio (chalcedony, called ‘agate’ in English because of its mysterious swirling colours). Small items in this unique glass could fetch a high price. Further, although calcedonio was known in London, having been imported from Venice by glass seller, John Greene in 1668, it had clearly never been made in London before. It was this that tempted Sir Christopher Wren and his architectural assistant, Robert Hooke, both pillars of the new Royal Society, to go along to watch it being made; it is from Hooke’s diary that we learn this crucial information. Surprisingly, it is because da Costa was making calcedonio that the new “crystalline glass resembling rock crystal”, patented by Ravenscroft, was discovered.

Small calcedonio vase, 17th century.The swirling colours seen in calcedonio are achieved by a unique process found (in the early texts) only with the difficult-to-make copper red and pink glasses. Instead of the colouring agent being mixed in with the cold batch materials and then founded, as is common practice today, with calcedonio the mixture of colouring pigments is stirred into the clear molten glass after it has been brought to clear fluid perfection by the being founded for 12 hours. The crucial details are outlined in a mid-sixteenth century recipe book attributed to Angelo Barovier (or close relative) and published in 2001 by Moretti and Toninato. These authors compare a number of recipes for calcedonio described in several notebooks of the period. Most of the variation between the recipes resides in the preparation of the colouring salts and oxides. The final product is always achieved by stirring the colour mixture into the molten cristallo, usually containing added lead and sometimes tin as well.
(In discussing their recipe book these authors recognise (page 40) from the description by Robert Hooke of the materials being used that da Costa was a specialist jewel maker but failed to make the connection between calcedonio and English lead crystal.)
As mentioned above, the materials used for pastes and enamels described in the original recipe were of the highest quality, the base glass being compounded from powdered flints and best Syrian ash as the source of soda. Powdered flints were no problem for da Costa but Venice had a monopoly on Syrian ash, so some substitution was required, even in the Italian provinces. In England, the most probable was pure potash obtained by calcining tartar from wine barrels to which would have been added saltpetre following general English practice dating back to Sir Robert Mansell in the early years of the century. Saltpetre was added to minimise discolouration of the glass by fumes from the furnace.

It is possible (although from the recipes described it seems unlikely) that Venetian lead glass was not immediately added to the cristallo frit at this early stage of the enterprise. However, we may imagine that Ravenscroft was admiring of the quality of the trial samples of molten glass taken before the colouring powder was added. Da Costa must then have told him that (as described in the original recipe) he could make an even better glass but it would be more expensive to make because of the special lead glass used, the extra time involved and, worst of all, that metallic lead that formed during founding destroyed the melting pots. (In the 1535 Manuscript of Montpellier on glass making,in recipe 134 lead glass is defined as "the base of the whole art of our glass making, because [...] you will never make good colours if you don't make them with glass of lead.) So, we may imagine that Ravenscroft, full of enthusiasm, replied “Well! let’s give it a try anyway” and English lead crystal was born.

Based on the original mid-16th century recipe the lead content in the final glass would have been approximately 14%. With the batch substitutions and the addition of saltpetre this may have been less - one sealed Ravenscroft sample I tested gave a value of just under 10%. Other tests on surviving samples of lead glass have given values of around 15% while crizzling is said to occurs in test batches if the lead content is 9% or less.The fact of the new glass crizzling and that it took another two years, including the proving period, shows first that it could not have been invented in the short time between when the Savoy glass furnace was built (May 9th, 1673) and Ravenscroft applied for a patent in early March 1674. Second, it also confirms that da Costa did not initially know how to make English lead crystal glass, or even that there was any profit in doing so.

The Importance of Saltpetre in the Batch

A discovery of no less importance was that with this new batch composition containing saltpetre, molten lead no longer formed during founding and attacked the pots (the complex process of oxidation and reduction was not resolved until the 19th century).
As well as the protection from fumes etc. already mentioned,there was a further benefit that has not previously been remarked upon; and that, as described by Robert Dossie in his Handmaid to the Arts (1764), is that the saltpetre eliminates the yellow cast caused by lead as mentioned above:-
"Salt-petre is made an ingredient of those compositions for transparent colourless glass where lead is used as a flux; for such glass, having, otherwise, a strong tinge of yellow from the phlogiston of the lead, requires, consequently, the destruction of the phlogiston, at least to a certain degree, in order to its being free from this tinge."A lead light baluster of c. 1695 showing the yellow tinge, possibly due to an absence of saltpetre in the batch. It also shows much fine seed suggesting that saltpetre may be involved in the clarifying process. The chemistry is quaint of the time but the statement is clear. Ravenscroft's sealed glasses are without any such tint and we can be sure that saltpetre was included in the batch by that time if not from the beginning.

This combination of improvements over the original continental lead silicate glass described above opened the way for the economically viable commercial exploitation of the discovery. On March 19th, 1674, just under a year after building the furnace, Ravenscroft was awarded his patent for this new glass resembling rock crystal.

What happened next was not simply what past accounts may lead the reader to believe. Da Costa probably initially continued to make bijouterie at the Savoy. He probably helped cure the crizzling problem that occurred with the new glass, with a new participant, Hawley Bishopp while negotiations with the Glass Sellers Company eventually led to a new glasshouse for tableware being set up at Henley on Thames, further up the river. Ravenscroft, however, had a much better plan to make a quick buck – the manufacture of mirror plates. About this time the French architect, Jules Hardouin-Mansart, was planning an upgrade for the Palace of Versailles with its famous Hall of Mirrors. A mirror was the new must-have for the rich and the Vauxhall plate glasshouse, founded by the Duke of Buckingham back in 1663 was thriving. But the value of a mirror depended on the clarity of the glass (a matter also emphasized by Robert Dossie). A greenish or blueish off-colour reflected an image distressing to the viewer and so markedly decreased its value. Ravenscroft’s water-white crystal was an ideal medium and could command the highest price.

Although the Duke of Buckingham had sold the glasshouse to John Bowles he had retained the monopoly on making mirrors. By July, 1674 Ravenscroft had negotiated with the Duke the remainder of his 7-year patent relating to the Vauxhall glasshouse. About this time, Pietro Rossetti, a Venetian glassmaking mirror expert, came to stay with Ravenscroft, but only for two weeks having probably been dispatched homewards by the ever-vigilant Venetian Ambassador. Then, in 1675, Ravenscroft was joined by John Baker, a glassmaker said to have a long history in the plate glass industry and who also had a glashouse in Chelsea. A new glasshouse was built in Vauxhall, on the river bank a short distance from the Duke’s old glasshouse now run by John Bellingham. By this time the crizzling problem that first affected the new lead crystal was probably overcome (said to have been overcome several months ago on June 3, 1676) and Baker began making mirror plates in quantity, some said to be as large as 35 to 40 inches in width. The competition clearly affected Bellingham who officially complained that “they made great quantities thereof and sold the same to “your Orator Bellingham’s damage”.

The problem seems to have been solved by natural events. John Baker died in 1679, the year that Ravenscroft abandoned his patent. Bellingham leased the glasshouse from Mrs Baker and presumably managed it until his death in 1700 when it became derelict. Meanwhile, the merits of English lead crystal for tableware had become apparent to the other London glassmakers who gradually abandoned the Venetian styles for which it was not well-suited in favour of the heavy baluster that became dominant by the end of the century.

A simple lead glass baluster goblet c. 1700. What finally happened to da Costa we do not know. When George’s brother, Francis took over the Savoy furnace he probably moved to Henley upon Thames.
Formica reappears in Ireland and, in 1775, probably having learned from da Costa of the new crystal glass, applies for Letters Royal to make it, now almost ten years after he had left Nijmegen.Reinier, with a new glasshouse in Stockholm was making some form of lead glass by about 1678. In London, probably as a result of the activities of the Royal Society, a Dr. Ludwell of Oxford made a form of glass, more resembling Pyrex than lead crystal, based on ingredients seen at the Savoy glasshouse by Robert Hooke while historian, Robert Plot gives the matter brief mention in his book on Oxfordshire including the building of the glasshouse at Henley on Thames.

For the collector the importance of the discovery (rather than deliberate invention) of English lead crystal glass is that it provides a clear starting point from when objects made by the English industry could be identified and confusion with European products eliminated. In America, problems remain over the distinction between some English imports and those of its own developing industry. But these are far less than if the discovery had never been made.

It was the unique combination of da Costa’s special expertise in making calcedonio and Ravenscroft’s “prepared commercial mind” and persistence in overcoming the crizzling problem that resulted in the chance discovery of English lead crystal.

The fully referenced version of this text is available in my book.

For more pictures and information see my Fitzwilliam English Glass History Trail, chapter 2.